WB1 8.11 Auto-RP

8.11 Auto-RP

• Remove all static RP settings and previous Accept-RP/Register configurations.
• Using a Cisco proprietary technology, configure R5 so that it advertises itself as the RP for all multicast groups.

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Auto-RP was the first protocol to automatically distribute RP information across
the multicast domain when using PIMv1. Auto-RP is a Cisco proprietary protocol,
and it was later “replaced” by the standards-based Bootstrap Router (BSR)
protocol with the advent of PIMv2. However, all Cisco routers still support Auto-
RP along with PIMv2 protocol.

Auto-RP defines two new concepts – Candidate RP (cRP) and Mapping Agents
(MA). cRP is any router that is willing to become an RP. You configure this router
using the command

ip pim send-rp-announce <Interface> scope <TTL> [group-list <Std-ACL>] [interval <seconds>].

The router will start sending UDP packets to the IP/Port 224.0.1.39/496 with the list of
groups serviced by this particular RP. The cRP announcements are originated
every 60 seconds by default, with the Time-to-Live field in the IP headers set to
<TTL> – a form of administrative scoping. The interface that you specify in this
command must be enabled for PIM and its IP address will be used as the RP’s IP
address. The list of groups is defined using the <Std-ACL> access-list. You
configure this list using standard access-list syntax, e.g.

access-list GROUPS permit 239.0.0.0 0.0.0.255
access-list GROUPS permit 232.0.2.0 0.0.0.255
access-list GROUPS deny 224.0.1.50 0.0.0.0

The Auto-RP code will convert wildcard masks into the prefix-lengths, so you
cannot use discontinuous masks. Next, the “deny” statements are interpreted as
a special type of negative group announcement. This means that all groups
matching this range should be treated as “dense” mode groups. We will discuss
how the routers interpret the mapping entries a bit later.

The cRP announcements are flooded across the network and reach special
routers called Mapping Agents. You configure these routers using the command

ip pim send-rp-discovery <Interface> scope <TTL> interval <Seconds>.

Mapping Agents listen on the standard address 224.0.1.39 and
collect all announcements from candidate RPs. After that, every mapping agent
compiles a resulting list of Group to RP mappings and starts sending “RP
discovery” messages to the special multicast address 224.0.1.40 port 496. The
discovery messages contain an amalgam of all information learned by the
mapping agents. Notice that if there are multiple MAs in the network, they will
hear each other, and then all of them, except the one with the highest IP
address, will cease sending discoveries.

When building a discovery message, an MA will follow a few simple rules:

1) If there are two announcements with the same group range but different RPs,
the MA will select the announcement with the highest RP IP address.
2) If there are two announcements, where one group is a subset of another, but
the RPs are different, both will be sent.
3) All other announcements are grouped together without any conflict resolution.

All regular routers join the multicast group 224.0.1.40 and listen to the discovery
messages. Based on their content, they populate their Auto-RP cache and learn
about Group-to-RP mappings. The cache contains both “negative” and “positive”
entries. When looking for an RP, Auto-RP code will first scan through negative
entries. If a match is found, the group is considered to be dense. Note that RP
information for the negative entries will be effectively ignored. If the group is not
found in the “negative” list, the code looks it up in the positive list. Since every
group in the list is bound to a particular RP, there could be conflicts when
multiple RPs try to service overlapping group ranges. The receiving router uses
the longest-match rule to resolve all conflicts: if there are multiple matches, only
the one with the longest prefix length is selected.

Note that “negative” statements could be defined at any cRP and affect ALL
routers in the multicast region. For example if a single group list would contain
deny any statement, then all groups will be treated as dense, even if there are
“positive” entries

The last thing to discuss about Auto-RP is how the multicast groups 224.0.1.39
and 224.0.1.40 are propagated across the network. Since there is no explicit RP
information for these groups, they must use dense mode forwarding. This
requires the use of pim sparse-dense-mode on all interfaces within the
multicast domain. As it has been discussed before, this is not the safest thing to
do in a large-scale network. You may want to use the no ip dm-fallback
global command in such situations or use the Auto-RP Listener feature,
discussed in a separate task.

Note that you may use PIM SM mode along with Auto-RP if you define a static
RP value for the Auto-RP groups (224.0.1.39 and 224.0.1.40). This will require
you to use the override option when defining the static RP. By default, Auto-
RP announcements override a statically configured RP. If you want them to
persist, use the override keyword along with the ip pim rp-address
command.

You may be asking one question – why is there a need for a Mapping Agent?
Couldn’t candidate RPs just broadcast themselves to all routers and the latter
learn/elect the best RPs directly? This is possible, but might result in different
routers electing different RPs for the same group ranges. Some routers may miss
announcements of a particular candidate RP due to network outages or RP
failures. Therefore, cRP information should be collected at one single point
before being disseminated to the multicast routers.

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R1, R3, R4, R5, R6, SW2, SW3, SW4:
no ip pim rp-address 150.1.5.5

R5:
no ip pim accept-register route-map ACCEPT_REGISTER
no ip pim accept-rp 150.1.5.5 ALLOWED_GROUPS
!
interface Loopback0
ip pim sparse-dense-mode
!
ip pim send-rp-announce Loopback 0 scope 10
ip pim send-rp-discovery loopback 0 scope 10

SW2:
no ip pim accept-rp 150.1.5.5 ALLOWED_GROUPS

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Rack1R1#show ip pim rp mapping
PIM Group-to-RP Mappings
Group(s) 224.0.0.0/4
  RP 150.1.5.5 (?), v2v1
    Info source: 150.1.5.5 (?), elected via Auto-RP
         Uptime: 00:01:01, expires: 00:02:57
Rack1R1#

Rack1R3#show ip pim rp mapping
PIM Group-to-RP Mappings
Group(s) 224.0.0.0/4
  RP 150.1.5.5 (?), v2v1
    Info source: 150.1.5.5 (?), elected via Auto-RP
         Uptime: 00:01:54, expires: 00:02:03
Rack1R3#

Rack1R4#show ip pim rp mapping
PIM Group-to-RP Mappings
Group(s) 224.0.0.0/4
  RP 150.1.5.5 (?), v2v1
    Info source: 150.1.5.5 (?), elected via Auto-RP
         Uptime: 00:02:13, expires: 00:02:44
Rack1R4#

Rack1R5#show ip pim rp mapping
PIM Group-to-RP Mappings
This system is an RP (Auto-RP)
This system is an RP-mapping agent (Loopback0)
Group(s) 224.0.0.0/4
  RP 150.1.5.5 (?), v2v1
    Info source: 150.1.5.5 (?), elected via Auto-RP
         Uptime: 00:02:33, expires: 00:02:25
Rack1R5#

Rack1R6#show ip pim rp mapping
PIM Group-to-RP Mappings
Group(s) 224.0.0.0/4
  RP 150.1.5.5 (?), v2v1
    Info source: 150.1.5.5 (?), elected via Auto-RP
         Uptime: 00:03:01, expires: 00:02:57
Rack1R6#

Rack1SW2#show ip pim rp mapping
PIM Group-to-RP Mappings
Group(s) 224.0.0.0/4
  RP 150.1.5.5 (?), v2v1
    Info source: 150.1.5.5 (?), elected via Auto-RP
         Uptime: 00:03:20, expires: 00:02:37
Rack1SW2#

Rack1SW3#show ip pim rp mapping
PIM Group-to-RP Mappings
Group(s) 224.0.0.0/4
  RP 150.1.5.5 (?), v2v1
    Info source: 150.1.5.5 (?), elected via Auto-RP
         Uptime: 00:03:32, expires: 00:02:27
Rack1SW3#

Rack1SW4#show ip pim rp mapping
PIM Group-to-RP Mappings
Group(s) 224.0.0.0/4
  RP 150.1.5.5 (?), v2v1
    Info source: 150.1.5.5 (?), elected via Auto-RP
         Uptime: 00:03:47, expires: 00:02:10
Rack1SW4#